Conventionally, a relay drive unit, as disclosed in, for example, Japanese Patent Laid-Open No. 2004-178967 (JP '967), is used for controlling a relay in an in-vehicle electronic control unit. The relay drive unit drives a relay to switch a power supply from a battery on/off for a motor drive unit. The relay drive unit may include a coil voltage detecting unit, a transistor, and a relay drive instruction unit. The coil voltage detecting unit detects a voltage between terminals of a relay coil as a coil voltage, and the transistor turns the power supply from the battery on/off. The relay drive instruction unit turns the transistor on/off by outputting a pulse width modulation signal (i.e., a PWM signal) to the transistor, where the transistor is turned on/off according to the coil voltage detected by the coil voltage detecting unit.
When the coil voltage changes according to the change of the battery voltage, the relay drive unit of JP '967 changes the duty ratio of the PWM signal according to the coil voltage detected. In such manner, an electric current flowing in the coil is kept stable at a constant value, and the relay is kept in the ON state.
Another example of a relay drive unit is disclosed in, for example, Japanese Patent Laid-Open No. 2006-185811 (JP '811). In this example, the relay drive unit includes an initial power on circuit, a constant voltage power circuit, and an electric current keep circuit. The initial power on circuit turns on the relay by supplying an initial electric current based on the battery voltage when an external switch is turned on. The constant voltage power circuit generates a constant voltage for keeping a relay contact in an ON state after the relay has transited to an ON state. The electric current keep circuit keeps the constant voltage being supplied for the relay by using the constant voltage as a power source. Per the relay drive of JP '811, the voltage of the relay coil is fed back to the constant voltage power circuit based on the monitoring of the voltage of the relay coil, and a constant voltage is output from the constant voltage power circuit based on such feedback, so that an electric current flowing in the relay is kept to a constant amount.
However, in the relay drive unit of JP '967, the circuit for detecting the inter-terminal voltage of the relay coil is separate from the relay drive circuit. Therefore, the production cost of the relay drive unit is increased. Further, in the relay drive unit of JP '811, the circuit for keeping the constant voltage of the relay coil is complex by using many parts, also leading to an increase of production cost.